Brake-by-wire (BBW) and steer-by-wire (SBW) systems for motor vehicles have reached at least prototype stage of development with some systems being implemented into production vehicles. In BBW systems, braking of each wheel may be controlled by independently operating, usually electromechanical or electro-hydraulic, actuators. Consequently, failure modes that may occur in these systems are different from those experienced in conventional (e.g., hydraulic or pneumatic) brake systems. As BBW systems generally include some level of redundancy in measurements, algorithms have been developed for detection and identification of failure modes.
In SBW systems, steering angle of front (or rear) wheels may be controlled by actuators. In true SBW or Active Rear Steer (ARS) systems, this may be achieved without direct mechanical link to the hand-wheel angle. In Active Front Steer (AFS) systems, a steering correction may be applied to the front wheels in addition to operator steering input. One advantage of these systems is that a corrective term can be applied to the front (or rear) steering angle independently of the vehicle operator.
One strategy for dealing with brake failure, which is consistent with current approach used for conventional (hydraulic) brake systems, is to disable the brake actuator on the opposite side of vehicle. This is usually done at the wheel that is diagonally opposite of the failed brake. This reduces the yaw moment applied to vehicle as a result of side-to-side imbalance in brake forces. However, it also reduces the deceleration rate and increases the stopping distance of the vehicle. As such, it would be desirable to provide a strategy for dealing with brake failure without significantly reducing the deceleration rate and increasing the stopping distance of the vehicle.
Another strategy for dealing with brake failure involves redistributing brake force lost from the failed brake equally to the remaining functioning brake(s). Such a strategy is disclosed in U.S. Pat. No. 6,062,657 issued to Dimasi on May 16, 2000. The Dimasi patent teaches a brake failure compensation system and method for a plurality of vehicle cars (e.g., train cars) articulately connected to one another. One shortcoming of this strategy relates to an inability to properly handle side-to-side imbalances that arise from the yaw moment. The generation of a larger yaw moment pulls vehicle to one direction during brake failure, requiring steering correction to maintain the desired path. In extreme conditions, the vehicle may spin out unless the vehicle operator is able to counter the brake force imbalance by steering quickly in an opposing direction. As such, it would be desirable to provide a strategy for dealing with brake actuator failure while minimizing side-to-side imbalances arising from the yaw moment.
Therefore, it would be desirable to provide a strategy for braking a vehicle during brake failure that overcomes the aforementioned and other disadvantages.